An extended x-ray absorption fine structure study of hydrogen storage in Ti-Hf-Ni alloys

The effect of hydrogenation on the local structure of a Ti40Hf40Ni20 crystalline alloy was studied by means of the extended x-ray absorption fine structure. The samples were loaded to different hydrogen-to-metal ratios, H/M, between 1.2 and 1.7. The desorption process was also investigated by studying the local order in an alloy charged to H/M = 1.85 then desorbed to H/M = 0.8. For the non-hydrogenated alloys, the local order was very similar in Ti-Hf-Ni and Ti-Zr-Ni alloys, quasicrystal or crystal. With increasing hydrogen concentration, all of the mean interatomic distances increased, except for the Ni-Hf (Hf-Ni) ones, which show an anomalous behaviour, decreasing with increasing H/M. This leads to a remarkable inversion of the atomic subshells of titanium and nickel in the first environment of hafnium atoms around H/M = 1.2. The increase of distances is a maximum for the Ti-Ti and the Ti-Hf correlations, which is consistent with the hydrogen atoms sitting preferentially near titanium and hafnium atoms. Similar conclusions were obtained previously from a study of Ti-Zr-Ni alloys